Methods for growing and harvesting carbon nanotubes

ABSTRACT

A method for directly growing carbon nanotubes, and in particular single-walled carbon nanotubes on a flat substrate, such as a silicon wafer, and subsequently transferring the nanotubes onto the surface of a polymer film, or separately harvesting the carbon nanotubes from the flat substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication Ser. No. 60/694,545, filed Jun. 28, 2005, in accordance with35 U.S.C. §119(e), which application is hereby expressly incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention is related to the field of catalysts for producing carbonnanotubes and methods of their use, and more particularly, but not byway of limitation, single-walled carbon nanotubes, and to methods toproduce polymers and products comprising carbon nanotubes.

Carbon nanotubes (CNTs) are seamless tubes of graphite sheets with fullfullerene caps which were first discovered as multi-layer concentrictubes or multi-walled carbon nanotubes (MWNTs) and subsequently assingle-walled carbon nanotubes (SWNTs) in the presence of transitionmetal catalysts. Carbon nanotubes have shown promising applicationsincluding nanoscale electronic devices, high strength materials,electron field emission, tips for scanning probe microscopy, and gasstorage.

Generally, single-walled carbon nanotubes are preferred overmulti-walled carbon nanotubes for use in these applications because theyhave fewer defects and are therefore stronger and more conductive thanmulti-walled carbon nanotubes of similar diameter. Defects are lesslikely to occur in SWNTs than in MWNTs because MWNTs can surviveoccasional defects by forming bridges between unsaturated carbonvalances, while SWNTs have no neighboring walls to compensate fordefects.

SWNTs in particular exhibit exceptional chemical and physical propertiesthat have opened a vast number of potential applications.

However, the availability of CNTs and SWNTs in particular in quantitiesand forms necessary for practical applications is still problematic.Large scale processes for the production of high quality SWNTs are stillneeded, and suitable forms of the SWNTs for application to varioustechnologies are still needed. It is to satisfying these needs that thepresent invention is directed.

Previous U.S. patents and applications directed to catalysts and methodsof producing carbon nanotubes, including U.S. Pat. Nos. 6,333,016,6,413,487, U.S. Published Application 2002/0165091 (U.S. Ser. No.09/988,847), and U.S. Published Application 2003/0091496 (U.S. Ser. No.10/118,834), are hereby expressly incorporated by reference herein intheir entireties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows (A) a micrograph of carbon nanotubes grown on a flatsubstrate, and (B) a Raman spectra of the carbon nanotubes of (A).

FIG. 2 shows (A) a micrograph of carbon nanotubes grown on a flatsubstrate, and (B) a Raman spectra of the carbon nanotubes of (A).

FIG. 3 shows (A) a micrograph of carbon nanotubes grown on a flatsubstrate, and (B) a Raman spectra of the carbon nanotubes of (A).

FIG. 4 is a schematic drawing showing the steps (A, B, C) of polymertransfer of nanotubes from a flat surface.

FIG. 5 shows Raman spectra of nanotubes at three stages of polymertransfer of nanotubes from a flat surface.

FIG. 6 shows SEM images of SWNTs produced on silicon wafers withcatalyst solution of different concentrations: (a) 0.38, (b) 0.19%, (c)0.02%. Concentration is in total metal weight. These loadings correspondto metal areal loadings of (a) 16 micrograms/cm², (b) 8 micrograms/cm²,and © 0.8 micrograms/cm².

FIG. 7 shows structural characterization of VSWNTs: (a) TEM images ofVSWNT material removed off the silicon wafer without any purification.(b) Raman spectra of as-produced VSWNTs with excitation lasers ofwavelengths 633 nm (solid line) and 488 nm (dashed line).

FIG. 8 shows side-by-side comparison of SEM pictures (left column) ofSWNTs with AFM images (right column) of corresponding silicon waferswith catalyst solution of different concentrations: (a) 0.02% wt, (b)0.19% wt, (c) 0.38% wt. Concentration (% wt) refers to total metalweight. AFM images were obtained after the silicon wafers were calcinedin oven at 500° C. All the scale bars in SEM images are 500 nm and thewidths of the AFM 3-D squares are 1 μm except the one in panel a2, whichis 5 μm.

FIG. 9 shows schematic diagram illustrating the proposed growthmechanism of 3-order structures of V-SWNTs: from left to right is1^(st)-order structure of a single tube, 2^(nd)-order structure of abundle, and 3^(rd)-order structure, which can be 2D (xy) grass or 1D (z)forest.

FIG. 10 shows SEM images of orderly arrays of SWNTs patterned by fastdrying process (a) and grid-masked sputter coating (b). Images weretaken with lower (1) and higher magnification (2).

FIG. 11 is a schematic representation of a continuous process of growingand harvesting carbon nanotubes from flat surfaces.

FIG. 12 is a schematic representation of an alternative continuousprocess of growing and harvesting carbon nanotubes from flat surfaces.

FIG. 13 shows SEM images of V-SWNT as it is detached from the flatsurface directly in air. (A) SEM image of V-SWNT attached to the flatsurface; (B) SEM image of V-SWNT detached from the flat surface.

FIG. 14 shows TEM image of V-SWNT showing the absence of metalimpurities.

FIG. 15 shows XANES spectra of V-SWNT at different angles of incidencewith respect to top surface of V-SWNT.

FIG. 16 shows graphs with experimental and fitted data of σ* and π* peakintensity.

FIG. 17 shows SEM micrographs of top (A) and side (B) views of a typicalV-SWNT sample by SEM.

FIG. 18 shows SEM images of V-SWNTs obtained for a series of reactiontime period. The scale bars in those images are 1 μm for 0, 30, 60seconds and 3 minutes, 2 μm for 10 minutes and 5 μm for 30 minutes.

FIG. 19 shows Raman spectra of V-SWNT obtained for a series of reactiontime period: 0.5 minutes, 3 minutes and 10 minutes from bottom to top.These 3 curves are normalized with respect to Si band at 520 cm⁻¹. Theinset is G band of V-SWNT obtained for 0.5 minutes (solid line) and 10minutes (dashed line) respectively when normalized with respect to Gband.

DESCRIPTION OF THE INVENTION

The present invention contemplates methods of producing CNT-bearingfilms, and preferably SWNT bearing-films, on flat surfaces (flatsubstrates) such as silicon wafers which have small amounts of catalyticmetal, e.g., cobalt and molybdenum, disposed thereon.

The carbon nanotube-polymer film compositions produced herein can beused, for example as, electron field emitters, fillers of polymers inany product or material in which an electrically-conductive polymer filmis useful or necessary for production. CNTs grown on flat surfaces canbe removed from the flat surface by different means (including, but notlimited to, peeling off as in the examples, shearing, sonication, andchemical etching of the flat surface) resulting in high purity CNTs thatcan be used for any CNT application. The flat surface-CNT material couldalso be used in applications such as sensors, interconnects,transistors, field emission devices, and other devices.

The flat substrates of the present invention include substrates havingcontinuous (non-particulate) surfaces which may be completely flat(planar) or may have a curvature including convex and concave surfacesand surfaces which have one or more trenches therein. It may alsoexhibit some roughness, which is small relative to the macroscopic scaleof the substrate.

Materials having flat surfaces contemplated for use as flat substratesor support material for the catalysts described herein, may include ormay be constructed from (but are not limited to): wafers and sheets ofSiO₂, Si, organometalic silica, p-or n-doped Si wafers with or without aSi₂ layer, Si₃N₄, Al₂O₃, MgO, quartz, glass, oxidized silicon surfaces,silicon carbide, ZnO, GaAs, GaP, GaN, Ge, and InP, sheets of metal suchas iron, steel, stainless steel, and molybdenum and ceramics such asalumina, magnesia and titania.

The catalytic materials used in the present invention are prepared inone embodiment by depositing different metal solutions of specificconcentrations upon the flat substrate (e.g., a silicon wafer). Forexample, Co/Mo catalysts can be prepared by impregnating various siliconwafers with aqueous solutions of cobalt nitrate and ammoniumheptamolybdate (or molybdenum chloride) to obtain the bimetalliccatalysts of the chosen compositions (see U.S. Pat. No. 6,333,016, theentirety of which is hereby expressly incorporated by reference herein).The total metal loading is preferably from 0.001 to 1000 mg/sq cm. Afterdeposition of the metal, the catalytic flat substrates are preferablyfirst dried in air at room temperature, then in an oven at 100° C.-120°C. for example, and finally calcined in flowing air at 400° C.-600° C.

Carbon nanotubes can be produced on these catalytic substrates indifferent reactors known in the art such as packed bed reactors,structured catalytic reactors, or moving bed reactors (e.g., having thecatalytic substrates carried on a conveying mechanism such as in thesystems described for example in more detail in Example 6).

The catalytic flat substrates may optionally be pre-reduced (e.g., byexposure to H₂ at 500° C. or, for example, at a temperature up to thereaction temperature) before the catalytic flat substrate is exposed toreaction conditions. Prior to exposure to a carbon containing gas (e.g.,CO), the catalytic flat substrate is heated in an inert gas (e.g., He)up to the reaction temperature (600° C.-1050° C.). Subsequently, acarbon-containing gas (e.g., CO) or gasified liquid (e.g., ethanol) isintroduced. After a given reaction period ranging preferably from 1 to600 min, the catalytic flat substrate having CNTs thereon is cooled downto a lower temperature such as room temperature.

For a continuous or semi-continuous system, the pretreatment of thecatalytic flat substrate may be done in a separate reactor, for example,for pretreatment of much larger amounts of catalytic flat substratewhereby the catalytic flat substrate can be stored for later use in thecarbon nanotube production unit.

In one embodiment of the invention, the catalytic flat substrateselectively produces SWNTs by the disproportionation of CO(decomposition into C and CO₂) in a preferred temperature range of700-950° C. (see U.S. Ser. No. 10/118,834, which is hereby expresslyincorporated by reference herein in its entirety).

The catalytic precursor solutions used for applying catalytic coatingsto the flat substrates of the present invention preferably comprise atleast one metal from Group VIII, Group VIb, Group Vb, or rhenium ormixtures having at least two metals therefrom. Alternatively, thecatalytic precursor solutions may comprise rhenium (Re) and at least oneGroup VIII metal such as Co, Ni, Ru, Rh, Pd, Ir, Fe and/or Pt. TheRe/Group VIII catalyst may further comprise a Group VIb metal such asCr, W, or Mo, and/or a Group Vb metal, such as Nb. Preferably thecatalytic precursor solutions comprise a Group VIII metal and a GroupVIb metal, for example, Co and Mo.

Where used herein, the phrase “an effective amount of acarbon-containing gas” means a gaseous carbon species (which may havebeen liquid before heating to the reaction temperature) present insufficient amounts to result in deposition of carbon on the catalyticflat surfaces at elevated temperatures, such as those described herein,resulting in formation of CNTs thereon.

As noted elsewhere herein, the catalytic flat substrates as describedherein include a catalytic metal composition deposited upon a flatsupport material.

The ratio of the Group VIII metal to the Group VIb metal and/or Reand/or Group Vb metal in the catalytic materials may affect the yield,and/or the selective production of SWNTs as noted elsewhere herein. Themolar ratio of the Co (or other Group VIII metal) to the Group VIb orother metal is preferably from about 1:20 to about 20:1; more preferablyabout 1:10 to about 10:1; still more preferably from 1:5 to about 5:1;and further including 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2, 1:1, 2:1,3:1, 4:1, 5:1, 6:1, 7:1, 8:1, and 9:1, and ratios inclusive therein.Generally, the concentration of the Re metal, where present, exceeds theconcentration of the Group VIII metal (e.g., Co) in catalytic precursorsolutions and catalytic compositions employed for the selectiveproduction of SWNTs.

The catalytic precursor solution is preferably deposited on a flatsupport material (substrate) such as a silicon wafer as noted above orother flat materials known in the art and other supports as describedherein as long as the materials have a flat surface, as describedherein. Preferably, the catalytic precursor solution is applied in theform of a liquid precursor (catalyst solution) over the flat substrate.

Examples of suitable carbon-containing gases and gasified liquids whichmay be used herein include aliphatic hydrocarbons, both saturated andunsaturated, such as methane, ethane, propane, butane, hexane, ethylene,and propylene; carbon monoxide; oxygenated hydrocarbons such as ketones,aldehydes, and alcohols including ethanol and methanol; aromatichydrocarbons such as toluene, benzene and naphthalene; and mixtures ofthe above, for example carbon monoxide and methane. Thecarbon-containing gas may optionally be mixed with a diluent gas such ashelium, argon or hydrogen or gasified liquid such as water vapor.

The preferred reaction temperature for use with the catalyst is betweenabout 600° C. and 1200° C.; more preferably between about 650° C. and1000° C.; and most preferably between 750° C. and 900° C.

In one embodiment, SWNTs may comprise at least 50% of the total CNTproduct produced on the catalytic flat substrates. Furthermore, SWNTsmay comprise 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 97.5%, 98% or 99%of the total CNT product.

In an alternate embodiment, MWNTs may comprise at least 50% of the totalCNT product. Furthermore, MWNTs may comprise 60%, 65%, 70%, 75%, 80%,85%, 90%, 95%, 97.5%, 98% or 99% of the CNT product.

In an alternate embodiment, double-walled CNTs may comprise at least 50%of the total CNT product. Furthermore, double-walled CNTs may comprise60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 97.5%, 98% or 99% of the CNTproduct.

In other embodiments, the CNT product may comprise a mixture of SWNTs,double-walled CNTs, and MWNTs.

While the invention will now be described in connection with certainpreferred embodiments in the following examples so that aspects thereofmay be more fully understood and appreciated, it is not intended tolimit the invention to these particular embodiments. On the contrary, itis intended to cover all alternatives, modifications and equivalents asmay be included within the scope of the invention. Thus, the followingexamples, which include preferred embodiments will serve to illustratethe practice of this invention, it being understood that the particularsshown are by way of example and for purposes of illustrative discussionof preferred embodiments of the present invention only and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of formulation procedures as well as ofthe principles and conceptual aspects of the invention.

EXAMPLE 1 Growth and Harvest of SWNTs

(1) Catalytic Precursor Solution (to Prepare the Catalytic Composition)

-   -   Cobalt salt solution: dissolve 0.3100 g cobalt nitrate in a        solvent such as isopropanol to make the total weight equal to        23.75 g., resulting in a Co concentration of 0.0442 mmol/g.    -   Molybdenum salt solution: add 1 g Dl (deionized) water to 0.9058        g molybdenum chloride under a hood, shaking well to make sure        all the molybdenum chloride is dissolved to form a brown        solution. Dilute the solution with a solvent such as isopropanol        to 25 g resulting in a Mo concentration of 0.1326 mmol/g.

Mix the Co and Mo solutions in equivalent weight and add 5% of a wettingagent such as tetraethylorthosilicate or other solvent described below.Other catalytic metals may be used as indicated above, including thosein Group VIII, Group VIb, Group Vb, and Re. Solvents which may be usedto dissolve the catalytic metal components include, but are not limitedto, methanol, ethanol, isopropanol, other alcohols, acetone, otherorganic solvents, acids, and water, depending on the solubility of themetal precursors and stability of the wetting agent.

Other wetting agents include, but are not limited to, silicates,silanes, and organosilanes, including polysiloxanes, polycarbosilanes,organosilazanes, polysilazanes, alkoxide-derived siloxanes,alkyl-cyclosiloxanes, alkyl-alkoxy-silanes, poly-alkyl-siloxanes,amino-alkyl-alkoxy-silanes, and alkyl-orthosilicates.

A catalyst stabilizer may be included and can be selected from the groupincluding, but not limited to: silicates, silanes, and organosilanesincluding polysiloxanes, polycarbosilanes, organosilazanes,polysilazanes, alkoxide-derived siloxanes, alkyl-cyclosiloxanes,alkyl-alkoxy-silanes, poly-alkyl-siloxanes, amino-alkyl-alkoxy-silanes,or alkyl-orthosilicates, as well as organotitanates, such as titaniumalkoxides or titanoxanes; organic aluminoxy compounds, organozirconates,and organomagnesium compounds (including Mg alkoxide).

The catalytic precursor solution (e.g., Co, Mo) can be prepared and usedimmediately, or prepared and stored for later use.

(2) Deposition of Catalytic P precursor Solution (e.g., Co/Mo) onSilicon Wafer

(Flat Substrates)—DSD (Drop-Spread-Dry) Process.

The deposition process in this example comprised dropping a small amountof the catalytic precursor solution onto the flat substrate. Thesolution (coating) spread over the substrate to form a uniform layer onit and dried quickly forming the catalytic composition on the catalyticflat substrate.

Alternatively, the catalytic precursor solution may be applied to thesubstrate movable support system via spraying, coating, spin coating,dipping, screen printing, or other methods known in the art. Also, thedrying process can be done slowly, by letting the flat substrate rest atroom temperature and covered to keep a higher relative humidity andlower air circulation than in open air.

(3) Thermal Pretreatment of the Catalytic Flat Substrates

The Co/Mo silicon wafer (catalytic flat substrate) thus produced can befurther dried in an oven at 100° C. for 10 min, then calcined in air at500° C. (or 400° C.-600° C. for 15 min in a muffle.

The calcined catalytic flat substrate was placed in a 1 inch-diameterquartz reactor, parallel to the gas flow direction and reduced in 1,000standard cc/min (sccm) of pure H₂ at 500° C., with a heating ramp fromroom temperature to 500° C. in 40 min and held at that temperature for 5additional minutes. Then, the feed to the reactor was switched to pureHe and temperature raised to 750° C. at a rate of 10° C./min. Thecalcination can be done immediately after drying or after leaving thedried catalytic flat substrate in storage for several days. Thecalcination temperature may vary from 300° C. to 650° C. and thecalcination time from 1 to 30 min.

Alternatively, the reduction temperature can be varied between 400° C.to 850° C. and the reduction time from 1 to 30 min. The heatingprocedure can be either using a ramp from 1 to 100° C./min, or byintroducing the sample on a preheated zone.

(4) Production of SWNTS on the Catalytic Flat Substrate.

-   -   a) The reduced catalytic flat substrate was exposed to a 1,000        sccm flow of pure CO at 750° C. The reaction lasted for 30 min        under 15 psi of pure CO.    -   b) After reaction, the system was kept at the same temperature        for 30 min under flow of He and finally cooled to room        temperature under He.

The CO gas velocity can vary between 1 cm/min and 10 m/min (standardconditions); with flow regimes from laminar to turbulent. The flowpatterns can be altered by use of baffles or trenches. Alternatively,the feed could be selected from methane, ethane, ethylene, ethanol, orother materials as described elsewhere herein. Also, co-feeding gasessuch as water, oxygen, or hydrogen can be employed.

(5) Transfer of SWNTs from the Catalytic Flat Substrate to AnotherMedium

-   -   a) After step 4, a mixture of polydimethylsiloxane (PDMS)        pre-polymer (Sylgard-184) and a cross-linking agent was        deposited on the surface of SWNT/catalytic flat substrate. The        weight ratio of PDMS to crosslinking agent was 10:1.    -   b) The wafer (i.e., the SWNT/catalytic flat substrate) with        polymer film was then sent to an oven to cure for 2 h at 60° C.        After cooling down, the resulting polymer film containing the        SWNTs, was peeled off the silicon wafer (catalytic flat        substrate). Raman characterization on both the Si wafer surface        and the polymer surface indicated that the transfer of SWNTs        into the polymer was practically (substantially) complete.

Examples of polymers which may be applied to the catalytic flatsubstrate having SWNTs thereon include, but are not limited to:polypropylene, polyethylene, polyacrylamide, polycarbonate, polyethyleneterephthalate (PET), polyvinylchloride, polystyrene, polyurethane,Teflon, Saran, polyacrylonitrile, polyvinylacetate, polyvinylalcohol,polymethyl methacrylate (PMMA), polyacrylates, polyguargum, polyesters,and polyamides such as nylon, as well as polymers formed in situ forexample by crosslinking pre-polymers applied to the nanotube-bearingcatalytic flat substrate (e.g., as shown in the example).

Similarly, the transferring medium could be a metal instead of apolymer. In this case, a metal film could be applied over the CNTs bydifferent methods, such as sputtering or evaporation. The metal filmcould subsequently be welded to another metal to make electricalcontacts, change surface properties, change heat conduction, and fluiddynamic properties, for example.

The transfer of CNTs from the catalytic flat substrate to thetransferring medium can adopt two possible configurations. In the first,the SWNTs are completely embedded in the matrix applied to the catalyticflat substrate and in the second, the transferring medium only covers afraction of the CNT structure and after the transfer, part of the CNTsremain exposed. The schematic shown below illustrates a brush-likestructure. However, the concept is not limited to this particularstructure, but applies to any other in which a portion of the CNTsremains exposed.

EXAMPLE 2 (A) Growth of SWNTs on Catalytic Flat Substrates FormingTwo-Dimensional Arrays

Effects of Gas Pressure on SWNT Density on the Catalytic Flat Substrate.

SWNTs were grown on the Co-Mo/Si wafer surface for 30 min at 750° C.,under different CO pressures. The catalytic flat substrates (wafers)were prepared following the recipe described in Example 1.

The resulting CNT structures, as observed by scanning electronmicroscopy (SEM) are illustrated in FIGS. 1 and 2. FIG. 1A shows growthof SWNTs at a lower pressure (15 psig) and shows a lower density ofSWNTs than in FIG. 2A, which showed a higher SWNT density obtained athigher pressure (80 psig). The corresponding Raman spectra (FIGS. 1B and2B) give clear evidence for the presence of SWNTs; that is, strongbreathing mode bands (at 200-300cm⁻¹), characteristic of SWNT), sharp Gbands (1590 cm⁻¹) characteristic of ordered carbon in sp2 configuration,and low D bands (1350 cm⁻¹), characteristic of disordered carbon in sp3configuration.

(B) Effects of Co/Mo Concentration on SWNT Density on the Catalytic FlatSubstrate

SWNTs were grown for 30 min under CO (P=15 psig) at 750° C. over twosurfaces having different loadings of Co/Mo catalytic metal.

In FIG. 1, the Co/Mo metal loading on the Si wafer was 16 mg/sq cm. TheSWNTs grown thereon had a low density as shown in FIG. 1A.

In FIG. 3, the Co/Mo metal loading on the Si wafer was 32 mg/sq cm. TheSWNTs grown thereon had a high density as shown in FIG. 3A.

As for the results in (A), Raman analysis (FIGS. 1B and 3B) clearlyshows the presence of SWNTs, with strong breathing mode bands (at200-300cm⁻¹), characteristic of SWNTs, a sharp G band (1590 cm⁻¹)characteristic of ordered carbon, and a low D band (1350 cm⁻¹),characteristic of disordered carbon.

EXAMPLE 3 (A) Transfer of SWNTs from the Catalytic Flat Substrate onto aTransfer Medium

In one embodiment of the invention, after the SWNTs are formed on thecatalytic flat substrate having the catalytic material thereon, they aretransferred to a transfer medium comprising a polymeric film or othermaterial (e.g., metal, ceramic, disordered film, elastomer, or carbon)deposited onto the catalytic flat substrate bearing the SWNTs (seeExample 1, Step 5). The transfer medium may have an adhesive materialthereon for enhancing adherence of the SWNTs thereto. A schematicrepresentation of the transfer process is shown in FIG. 4. FIG. 4A showsa flat substrate 10 having a catalytic surface 20 thereon, and a SWNTmass 30 present on the catalytic surface 20. A transfer medium 40,(e.g., a polymeric material) is applied to the catalytic surface 20 ofthe flat substrate 10 and upon the SWNT mass 30 (FIG. 4B), wherein thetransfer medium 40 is allowed to cure (if necessary), thereby causingtransfer and adherence or entrapment of the majority of the SWNT mass 30thereto. The transfer medium 40 and the SWNT mass 30 transferred theretocan be removed from the catalytic surface 20 (FIG. 4C). There is aresidual mass 50 of SWNTs left on the catalytic surface 20 after themajority of the SWNT mass 30 is removed therefrom.

(B) Characterization of Si Wafer and Polymer Surfaces Demonstrating theTransfer

FIGS. 5A-5C illustrate the different stages with the corresponding Ramanspectrum obtained after the SWNT mass is transferred. FIG. 5A showsRaman spectra of the SWNT mass 30 before transfer. This spectrum showsthe clear fingerprint of SWNT of high quality; that is, a strong G band,a weak D band and clear breathing mode bands. FIG. 5B shows a Ramanspectrum of the SWNTs on the polymer material 40 after forming a filmover the SWNT-containing Si wafer and peeling off. The Raman spectrumshows that a large fraction of the SWNT mass 30 has been transferredonto the polymer material 40. All the features characteristic of SWNTsare clearly seen on the polymer material 40. FIG. 5C shows a Ramanspectrum of a small residue of SWNTs left on the catalytic surface 20.As an internal calibration of the amount of SWNT on the surface of theSi wafer, one can note the relative intensities of the Si band and thatof carbon (e.g. G band at 1590 cm⁻¹).

EXAMPLE 4 (A) Influence of Catalyst Loading on the Morphology of SWNTFormations

Production of Vertically Oriented SWNTs on Flat Si Substrate.

Following the preparation procedures described in Example 1, catalyticprecursor solutions of varying metal concentration (0.001-3.8 wt %) wereprepared by dissolving salts of Co and Mo into isopropanol, whilekeeping a constant Co:Mo molar ratio of 1:3. The subsequent steps wereidentical to those in Example 1. The as-produced SWNT over the flatsubstrate were characterized by Raman spectroscopy, electron microscopy(SEM and TEM), and probe microscopy (e.g., AFM).

FIG. 6A-C illustrates the dramatic effect of catalyst loading on theresulting morphology of the SWNT formations. This reproducible trendundoubtedly demonstrates that the concentration of the catalyst solutionaffects the type of SWNT growth on the flat substrate. The SEM imagesclearly show that vertically aligned SWNTs (V-SWNT forest) almost 40micron in length were grown on the substrate impregnated with catalyticprecursor solution of 0.19 wt % total metal (Co-Mo) concentration (FIG.6B). By contrast, on wafers impregnated with catalytic precursorsolutions of 0.38 wt % (FIG. 6A) and 0.02 wt % (FIG. 6C), randomtwo-dimensional networks of SWNT (grass) were observed after reaction.The sample with higher metal concentration (0.38 wt %) produced a highernanotube density than the one with lower metal concentration (0.02 wt%), but neither of them resulted in vertical growth under the conditionsused in this example. The results indicate that there is an optimumsurface concentration of metals that results in vertical growth.Further, it was observed that “SWNT grass” grown with catalyticprecursor solution of the higher concentration (0.38 wt %) wasreasonably denser than that with catalytic precursor solution of lowerconcentration (0.02 wt %). Other concentrations having higher (up to 3.8wt %) or lower (0.001 wt %) loadings were studied, but none of themproduced vertically aligned SWNTs. In fact, the very high concentration(3.8 wt %) resulted in the formation of carbon fibers and multi-walledcarbon nanotubes, while the lowest concentration of catalyst (0.001 wt%) produced mostly scattered thin bundles of SWNTs.

(B) Structural Analysis

The structural characteristics of the nanotubes that make up the“forest” and “grass” formations observed by SEM were further evaluatedby Raman spectroscopy and TEM (FIG. 7). Raman spectroscopy is awell-known method for assessing the SWNT quality based on the relativeintensity of the D and G bands. TEM provides a direct identification ofthe nature of the carbon species deposited on the surface (i.e., SWNTs,MWNTs, amorphous or nanofibers). Raman spectra (as shown in FIG. 7B) ofthe as-produced V-SWNT forest were obtained with two excitation lasers(633 nm and 488 nm). The very low D/G ratio is consistent with SWNTs ofhigh quality with a low concentration of defective nanotubes ordisordered carbon species (e.g., nanofibers). At the same time, it iswell known that the frequency of the radial breathing mode bands (RBM)is inversely proportional to the nanotube diameter, according to theexpression ω_(RBM)=234/d_(SWNT)+10 [cm⁻¹]. The spectra for the V-SWNTsample obtained with three different lasers showed that the RBM brandscover a wide frequency range (from 130 cm⁻¹ to 300 cm⁻¹), whichcorresponds to a diameter range 0.8-1.9 nm, a distribution much broaderthan that typically obtained by the method of using Co-Mo catalystssupported on high-surface area silica. The broad distribution ofdiameters is also reflected in the convergence of the G⁻ and G⁺ featuresand broad base of the G band, in contrast with the sharper lines andmore pronounced separation of the G⁻ and G⁺ contributions for the G bandin the CoMoCAT material. The TEM observations of the V-SWNTs directlytaken from the substrate without any purification indicated the presenceof nanotubes of varying diameters (FIG. 7A) in agreement with the Ramanspectra (FIG. 7B). At the same time, TEM gives ample evidence of thepurity of the as-prepared V-SWNTs, free of other forms of carbon.

To explore the relationship between the metal loading on the flatsubstrate and the resulting morphology of the SWNT formations, weemployed atomic force microscopy (AFM), a powerful tool that provides3-dimensional profiles of surfaces. By investigating the morphology ofthe catalyst surface before the growth of nanotubes, we were able toidentify the optimum distribution of particles that result in V-SWNTforest. This analysis is illustrated in FIG. 8 that contrastshigh-magnification SEM pictures of the SWNT formations and AFM images ofcalcined catalyst/substrates for three different metal loadings. The AFMimage in FIG. 8 a 1 clearly shows that the catalyst particles generatedfrom the impregnating solution with the low metal concentration (0.02 wt%) were small and sparse. From this metal distribution, a similarlysparse formation of 2-dimensional SWNT grass was obtained (FIG. 8 a 2).In the case of intermediate metal concentration (0.19 wt %), the AFM inFIG. 8 b 1 evidences a dense population of nano-particles withrelatively uniform size. The average distance between these particleswas around 60-70 nm. It must be noted that TEM/EDXA analysis, as well sangle-resolved XPS analysis, showed that not all the Co and Mo addedremained exposed on the surface. Rather, a fraction of them becameburied in the layer of silica product that results from thedecomposition of the catalyst stabilizer and wetting agent during thethermal treatment. The SEM image of FIG. 8 b 2 shows that thisdistribution is successful in promoting the formation of V-SWNT forest.Interestingly, the density of nanotubes bundles is approximately thesame as the density of catalyst particles observed by AFM before thegrowth, which suggest that essentially every catalyst particle is activefor the production of nanotubes. By contrast, in the case of 0.38% wtmetal concentration as shown in FIG. 8 c 2, some bigger alloy particlesformed on the flat surface and possibly generated larger cobalt clusterswhich are not suitable for SWNT nucleation, while a small fraction of Coclusters with the optimum size to grow SWNT still remained between largeones. Thus, a thicker layer of SWNT grass was grown as shown in FIG. 8 c1.

In another observation of considerable note, it is observed that thecarbon deposits of a V-SWNT are observed directly perpendicular to thesurface, a random network of SWNT bundles is clearly evident over thetop of the forest, while the observation at different angles clearlyshows the well-aligned structure shown in FIG. 8 b 1. There is no doubtthat, in this case, the root-growth mechanism is operative. Therefore,the nanotube fraction that is observed on top of the forest has beenformed during the first stages of reaction, while the ordered(vertically-oriented) growth only occurs later, seemingly constrained bythe presence of the 2-dimensional crust formed in the initial stages.Initially, a layer of randomly arranged SWNT grows from a loose networkto a dense network (see FIG. 9). The density of this network depends onthe surface concentration of catalyst. With a low concentration ofcatalyst only a rather loose structure is formed. By contrast, in aregion with the proper catalyst density a dense nanotube array forms acrust (a horizontal layer of randomly-oriented carbon nanotubes) thatconstitutes a rather solid structure. This crust is later lifted up bythe growing nanotubes from the bottom (see FIG. 8 b 1). This is thereason why, while each individual nanotube is not perfectly straight andnot necessarily every nanotube has the same length, the overall foresthas a smooth top surface.

EXAMPLE 5 Production of Patterns of Vertically Oriented SWNT on FlatSubstrate

To further demonstrate the effect of the distribution of catalystparticles on the growth of SWNT on flat substrates, in addition to theuniform nanotube growth on uniform catalyst films described in Example4, patterned nanotube films were prepared by two different methods. Inone of the methods, the patterns occurred naturally and in the other,the patterning was controlled. Natural patterns were formed when a wetcatalyst thin film was dried at a fast drying rate. This method resultedin separate circular droplets of catalysts distributed over the Sisubstrate. By contrast, the controlled patterning was done by using amask and sputtering a film of Au-Pd on a previously formed homogeneouscatalyst film, prepared by slow drying and calcined in air. In this way,the fraction of catalyst covered by the Au-Pd alloy was selectivelydeactivated and no nanotube growth occurred on those regions. As aresult, the nanotube forest only grew from the remaining areas of activecatalyst. The as-produced SWNT over the catalyst/wafer werecharacterized by Raman spectroscopy, electron microscopy (SEM and TEM),and probe microscopy (AFM).

In the case of the natural pattern, fast drying in the air resulted inmicroscopic circular areas with varying concentration of catalyst. Forthe manual process, a TEM grid was used as a mask to protect thepreviously deposited Co-Mo catalyst. The fraction of the surface thatwas not covered by the grid was deactivated by a film of Au/Pd sputteredover the surface. The resulting patterned growth of V-SWNT obtained bythe two methods is illustrated in FIG. 10. FIG. 10 a 1 showsvolcano-shaped SWNT arrays on the substrate patterned by fast dryingmethod. A cross-sectional image of one of these volcanoes in highermagnification (FIG. 10A2) shows that they comprise vertically alignedSWNT near the edge of the ring, with 2-dimensional random arrangements(“grass”) in the middle part. Image (FIG. 10 a 2) shows parallel V-SWNTbars grown on activated catalyst area defined by TEM grid. Due todiffusion of Au-Pd from the edge into the space between grid andsurface, there is catalyst concentration gradient in the edge area. As aresult, the forest in this area bends towards the outside with the cruston the top extending continuously to the grass attached to substrate.

EXAMPLE 6 System for Continuous Production of Carbon Nanotubes on FlatSubstrates

In an alternative embodiment, the SWNTs may be grown on flat substrates100 (as defined elsewhere herein) in a continuous process, shown forexample in FIGS. 11 and 12. The flat substrates 100 are applied to aconveying assembly 110 such as a conveyor belt which can movedirectionally in a continuous manner. A catalytic precursor solution 120can be applied to the flat substrates 100 by a spraying mechanism 130 orby other application means including use of slot dies, rods, gravures,knife, over roll and reverse roll thereby forming catalytic flatsubstrates 100. As shown by example, furnaces 140, 150 and 160 can bepositioned such that the conveyor assembly 110 can convey the catalyticflat substrates 100 into a reaction zone 170 for sequentially calciningand reducing the catalytic flat substrates 100 to make them morecatalytically active then causing nanotube growth at varyingtemperatures. For example, at inlet 180, air may be input into thereaction zone 170 for calcining the catalytic flat substrates 100 infurnace 140, and H2 may be input at inlet 190 in the reaction zone 170for reducing the calcined catalytic flat substrates 100 in furnace 150.A carbon containing gas such as CO or ethanol may then be input into thereaction zone 170 at inlet 200 for supplying the carbon-containing gasfor the catalytic process of producing nanotubes in furnace 160. AsSWNT-bearing catalytic flat substrates 210 leave the furnace 160 withSWNTs 220 thereon, the SWNTs 220 can be harvested therefrom for exampleby a blade 230 or other methods not shown, including but not limited to,passing the belt through a tank and using sonication to promote releaseof the tubes, subjecting the nanotube field to a shear field (gas orliquid) or contacting the nanotube coated belt/web/plate with a tackymaterial. Gases may be removed or recycled from the reaction zone 170(to be reused or to have by-products removed therefrom) via outlets 185,195 and 205 for example. The catalytic flat substrates 100 which havebeen harvested of SWNTs 220 can then be removed from the conveyorassembly 110, for example, by passing them through a recycling unit 240or merely the catalytic composition 120 may be removed therefrom. Newflat substrates 100 can then be applied to the conveyor assembly 110 ornew catalytic precursor solution 120 may be applied to flat substrates100 which remain on the conveyor assembly 110.

Alternatively the SWNT-bearing catalytic flat substrates 210 could beused in a process described elsewhere herein for producing transfermedia (e.g., polymer films) having CNTs embedded therein (see Examples1-3).

The catalytic precursor solution 120 on the flat substrate 100 may bepatterned via printing, photolithography, or laser writing, for exampleafter spraying, or in lieu of spraying. The preparation and conditioningof the catalytic precursor solution 120 can be done offline.

Shown in FIG. 12 is an alternate version of the invention, similar tothe embodiment of FIG. 11, which comprises a plurality of flatsubstrates 100 a which are disposed on and secured to a conveyorassembly 110 a. A catalytic precursor solution 120 a, as describedabove, is applied to the flat substrates 100 a, via, for example, aspraying mechanism 130 a, or any other applicable method (before orafter the flat substrates 100 a are applied to the conveyor assembly 110a). The conveyor assembly 110 a transfers the catalytic flat substrates100 a into a furnace 140 a in a reaction zone 170 a which receives airvia inlet 180 a for calcining the catalytic flat substrates 100 a, whichare then transferred into furnace 150 a which receives a reducing gasfrom inlet 190 a for reducing the catalytic flat substrates 110 d whichare then transferred into furnace 160 a, which receives from thereaction zone 170 a and from an inlet 200 a a carbon-containing gas asdiscussed elsewhere for causing formation of SWNTs or CNTs on thecatalytic flat substrates 100 a to form SWNT-bearing catalytic flatsubstrates 210 a having SWNTs 220 a thereon. The SWNTs 220 a are thenremoved via a blade 230 a or by other means as discussed elsewhereherein. The catalytic flat substrates 100 a remain on the conveyorassembly 110 a and are used one or more additional times to form SWNTsbefore being recycled or removed and replaced. The catalytic flatsubstrates 100 a remaining on the conveyor assembly 110 a may beeventually treated or cleaned to remove the catalytic precursor solution120 a, and left on the conveyor assembly 110 a or may be removedentirely therefrom and replaced with fresh flat substrate 100 a,manually or automatically.

Other alternatives of a continuous belt which may be used includeroll-to-roll processes (unwind and rewind) or, continuous feeding offlat substrates or plates riding on a conveyor belt.

In alternate embodiments, an annealing step may be included to occurprior to release of the SWNTs from the SWNT-bearing catalytic flatsubstrates 210 or 210 a. Further, a functionalization step may occur,prior to release of the SWNTs, for example by radiation or plasma. Theresulting product of this process could be either the SWNTs themselvesor the SWNTs attached to the flat substrates.

EXAMPLE 7 Removal of SWNTs in Liquids

When the SWNTs produced on the catalytic flat substrates have to betransferred to a liquid medium, it is convenient to transfer themdirectly from the catalytic flat substrate to the liquid therebyavoiding intermediate steps. This transfer to a liquid medium can beachieved by dipping the SWNT-bearing catalytic flat substrate into asurfactant solution. In a simple experiment a 2 cm×1 cm piece ofV-SWNT-bearing silicon wafer was placed in a vial containing 7 ml of 1.3mmol/lt NaDDBS solution. Other surfactants can be used. After sonicatingthe sample for 1 min in a bath sonicator, the V-SWNT film detached andthe piece of silicon wafer was removed from the surfactant solution. Ifa good dispersion of the nanotubes in the liquid media is required, hornsonication can be used to break the nanotube bundles after the wafer hasbeen removed from the surfactant solution. Horn sonication of thesurfactant solution with the piece of wafer still inside may results inthe contamination of the sample with particles coming from thesubstrate.

Similar experiments were performed using other surfactants including:Sodium cholate, NaDDBS, CTAB and SDS; and other solvents including:isopropanol, chloroform, dichlorobenzene, THF and different amines.Alternative surfactants that could be used include, but should not belimited to: Surfynol CT324, Aerosol OS, Dowfax 2A1, Dowfax 8390,Surfynol CT131, Triton X-100, Ceralution F, Tween 80, CTAT and SurfonicL24-7. Other compounds such as polysaccharides (e.g., sodiumcarboxymethylcellulose) could also be used as a “surfactant” to changethe wettability of the surface or enhance the SWNT dispersibilty in theliquid media. As an alternative to surfactant solutions, other solventsthat can be used include, but are not limited to, alcohols, ketones,aldehydes, ethers, esters, alkanes, alkenes, aromatic hydrocarbons, andmixtures thereof. In some cases, bath sonication might not be requiredto remove the V-SWNT film from the flat substrate and the V-SWNT filmmight come off by itself after having been dipped in the solution orafter letting the liquid flow on top of the V-SWNT film. In some othercases, stirring or gentle agitation might be used as an alternative tobath sonication. An alternative method to transfer the V-SWNTs to aliquid medium consists of applying a liquid film on top of the V-SWNTmaterial, decreasing the temperature in order to freeze the liquid,mechanically removing the frozen liquid containing the V-SWNTs andmixing the frozen liquid containing the V-SWNTs with more liquid.Alternately, other CNTs, including non-vertically oriented SWNTs orMWNTs could be suspended using these methods.

EXAMPLE 8 Removal of SWNTs in Vacuum/Air

Nanotubes produced on a flat substrates may be removed from thecatalytic flat substrate directly in air using several simple methods,such as sweeping the surface with a soft spatula or blade or peeling ofthe film from the flat substrate (see FIG. 13). In general, it wasobserved that as the thickness of the nanotube film increased, it waseasier to remove the nanotube material. XPS analysis on the catalyticsubstrate used to grow the nanotubes and TEM and EDXA analysis of theV-SWNT material after it was detached from the catalytic substrateshowed that most of the catalytic metal (Co and Mo) remained on thecatalytic flat substrate while the nanotube material was free of metalimpurities (see FIG. 14), and thus did not substantially pull catalyticmaterial from the flat substrate during removal of the CNTs.

Alternatively, vibrations or a turbulent gas stream could be used toinduce the separation of the nanotube material from the flat substrate.The described methods could be used either in air, any other gas orvacuum. The described methods could be performed with the sample atambient temperature or after the sample has been heated above or cooledbelow ambient temperature.

EXAMPLE 9 Vertical Alignment of SWNTs During Growth on Flat SubstratesDue to the Formation of a Crust of Randomly Oriented Nanotubes

In this example, a description is given of the growth of verticallyaligned single-walled carbon nanotubes (or V-SWNT) on Co-Mo catalystsupported on a silicon flat substrate. The time evolution of the V-SWNTgrowth has been examined by scanning electron microscopy (SEM) andresonant Raman spectroscopy. A distinct induction period has beenidentified, during which a thin layer or 2-dimensional nanotube crust ofrandomly oriented SWNTs is formed on the substrate. The formation ofthis crust is followed by the concerted growth of a vertical nanotube“forest” whose height is controlled by the rigid nanotube crust thatholds the whole structure together. As a result, all the SWNTs areforced to grow in a substantially aligned fashion. Angle-resolved x-rayabsorption near edge structure (XANES) study of the full grown SWNTforest sample was obtained. The intensity of the C(1s)-to-π* andC(1s)-to-σ* transitions were quantified as a function of incidenceangle. A significant deviation of the experimental variation ofintensity with incident angle from the theoretical equation that onewould expect for perfectly oriented vertical nanotubes was observed atlow incidence angles. This deviation is in full agreement with thepresence of a crust of nanotubes on an upper surface of the nanotubeforest parallel to the upper surface of the substrate. Furthermore,several examples of different forms of SWNTs grown on a flat substrateare given to demonstrate the effect of a nanotube crust structure on theresulting topology of the SWNT forest.

(a) The catalyst of Co and Mo supported on Si wafer was prepared asdescribed in Example 1. After pretreatment, the wafer was placed in aquartz reactor, oriented parallel to the direction of the flowing gasand the SWNT growth was conducted as described above.

(b) The as-produced SWNTs over the catalyst/wafer (catalytic flatsubstrate) were characterized by Raman spectroscopy, electron microscopy(SEM and TEM), and angle-resolved X-ray near edge structure spectroscopy(XANES). The angle-resolved C K-edge XANES spectra were taken under UHVwith total electron yield (TEY) mode at the bending magnet beamline9.3.2 of Advanced Light Source (ALS) in Lawrence Berkeley NationalLaboratory (LBNL). The XANES data were collected at various anglesranging from θ=10° (“glancing geometry”) to θ=80° (“normal geometry”),where θ denotes the angle between the sample normal and the direction ofthe electric vector of the X-ray beam.

(c) Results of the XANES characterization: FIG. 15 shows the changes inXANES intensities at different incident angles for SWNT forest, where θis also the angle between the sample normal and the electric fieldvector of the X-ray beam. The pre-edge and post-edge in the XANESspectra were normalized to 0 and 1, respectively. Several characteristicpeaks can be identified in each set of the XANES spectra. The C K-edgeXANES spectra of SWNTs forest are quite similar to those of graphite, ashas also been reported by others. The spectra are characterized by asharp C—C π* transition near 285.4 eV, a sharp C—C σ* bond excitationnear 291.5 eV, two other σ* transitions from 292 to 298 eV and broad(σ+π) transitions from 301 to 309 eV. The position and width of theseresonances are typical of C—C single bond. Two small peaks in the287-290 eV region can be assigned to oxygenated surface functionalitiesintroduced while the SWNT forest were processed. These correspond to π*C═O and σ* C—O resonances. Following the method proposed by Outka andco-workers, the XANES spectra were fitted to a series of Gaussians, anarctangent step corresponding to the excitation edge of carbon, and abackground.

The presence of local order and texture in the SWNTs is observed in theangular dependence of the XANES of the SWNT forest. In forests ofnanotubes, the nanotubes are expected to point upwardly. Since thesynchrotron light is linearly polarized horizontally, the intensity ofthe π* transition is sensitive to the orientation of the π* orbital withrespect to the polarization vector. Thus, if the π* orbitals in thenanotube specimen are partially oriented with respect to the incidentphoton beam, a rotation of the specimen with respect to the incomingphoton will show a measurable angular dependence. At normal incidence,the electric field E is in the same cross section plane as the π*orbitals, and thus, the π* resonance peak will be the highest at thisangle, as opposed to at glancing angles. Conversely, when E is normal tothe surface, the field lies along the tube axis (along z) and isperpendicular to the plane of the π* orbitals, the intensity of the π*resonance is at its minimum. Specifically, there is an increase in theintensity of the π* resonance with increasing angle of X-ray beamincidence. The local contribution to the π* excitation XAS intensity isproportional to the square of the scalar product of the local normal andE. Obviously the π* resonance intensity is proportional to thesine-squared function of the incidence angle. A plot of the π*excitation vs the incidence angle shows a sine-squared dependence asshown in lower panel of FIG. 16.

However, a C—C σ* orbital orthogonal to the π* orbital will show anopposite trend. The σ* orbitals can be viewed as combination of twoperpendicular components, one is parallel to tube axis direction (σ*//),another along circumferential direction (also perpendicular to the tubeaxis, σ*^(⊥)), as seen in upper panel FIG. 16. The local contribution tothe σ* excitation XAS intensity at 291.5 eV is proportional to the sumof the squared scalar products of the two components and electricpolarization vector. With a simple calculation by accounting for all σ*contributions on the entire tube circumference, we found the intensityof the σ* bound resonance at 291.5 eV is proportional to (1+cos 2θ). Theintensity of the excitation has a cosine-square dependence with respectto the incidence angle as shown in FIG. 16. It is quite different fromthe observation of SWNT buckypaper, which resonances do not appear to bea systematic variation in intensity with incidence angle due therandomly distributed order of the tubes. Noticeable deviation of fitteddata from experimental data are observed for both σ* and π* transitionat low angles. Considering the mechanism discussed above, it is obviousto infer the presence of a crust with nanotubes oriented parallel to thesubstrate at the top of the V-SWNT. The presence of this crust isfurther supported by the SEM images of FIG. 17, which shows over theforest, SWNT oriented parallel to the surface in a random 2-dimensionalnetwork.

(d) Time-Evolution of the V-SWNT Growth.

In order to investigate the mechanism of formation of the V-SWNTstructure, we inspected the system at each stage of the growth process.The morphology of V-SWNT was observed by SEM after different reactiontimes. The results are summarized in the series shown in FIG. 18.Starting from a flat substrate with no carbon deposition, short SWNTbundles evolve after 30 seconds at some catalytically preferential spotswhich is probably because of geometrical and compositional difference inCo-Mo particles. But at this stage a continuous film of SWNTs has notbeen formed yet. During the following 30 seconds, almost all theparticles which are capable to nucleate caps that can grow into SWNThave been activated. Subsequently, the SWNTs grow thereby lifting thecaps. As a result (after 60 sec.), a thin layer of randomly orientedSWNTs has been woven. At 3 minutes, a uniform crust with very shortaligned SWNT bundles underneath can be clearly seen. The entangling ofSWNT bundles due to different growth rates and random orientationappears to stop at this stage. Instead, the growth rate of eachindividual bundle is averaged by the constraint imposed by the crust.Consequently (10 min. and 30 min.), a macroscopically uniform growth andalignment of the SWNTs occurs synchronically.

In addition to SEM, resonant Raman spectroscopy has been performed onthe time evolved V-SWNT samples. FIG. 19 shows the Raman spectrum ofV-SWNT produced in time period of 0.5 min, 3 min and 10 min. Ramanfeatures include G band at 1590 cm⁻¹, D band at 1340 cm⁻¹ and radialbreathing mode at 150˜300 cm⁻¹ which are typical for V-SWNT. The peak at520 cm⁻¹ is characteristic of inelastic scattering on silicon, whoseintensity is dependent on distance from the focal plane of laser whichis determined by height of V-SWNT and area covered by SWNT. In FIG. 19,three spectra are normalized to the Si band and the amount of V-SWNT canbe estimated by the size of the G band. It is clearly shown that theintensity of the G band increases with time. Interestingly, the shape ofG bands for the crust formed during the initial moments (0.5 min) andfor the V-SWNT formed after 10 min are different. As shown in the insetafter normalization, the V-SWNT sample shows convergent G− and G+ and abroader peak base in contrast with the sharper lines and more pronouncedseparation of the G− and G+ corresponding to the crust.

The data in this example indicate that (1) the growth of a V-SWNT forestrequires a very important step (referred to as the induction period),during which a thin layer (crust) composed of entangledrandomly-oriented SWNT bundles is formed initially as a guiding surfacefor the growth of vertically aligned SWNTS, and (2) after the nanotubecrust is formed, the subsequent growth of SWNT underneath is limited bythe crust, thereby causing all the nanotubes to have substantially thesame length.

Changes may be made in the construction and the operation of the variouscompositions, components, elements and assemblies described herein or inthe steps or the sequence of steps of the methods described hereinwithout departing from the scope of the invention.

1. A carbon nanotube structure, comprising: a catalytic flat substratehaving a catalytic surface; a first carbon nanotube layer comprisingrandomly-oriented carbon nanotubes; and a second carbon nanotube layercomprising vertically-oriented carbon nanotubes, and wherein the firstnanotube layer is disposed as an outer crust upon the second nanotubelayer such that the second nanotube layer is positioned between thefirst nanotube layer and the catalytic surface of the flat substrate. 2.The carbon nanotube structure of claim 1 wherein the carbon nanotubesare primarily single-walled carbon nanotubes.
 3. The carbon nanotubestructure of claim 1 wherein the catalytic flat substrate is preparedfrom a material selected from the group consisting of SiO₂, Si, p-dopedSi, n-doped Si, p-doped Si with a SiO₂ layer, n-doped Si with a SiO₂layer, Si₃N₄, Al₂O₃, MgO, quartz, glass, oxidized silicon surfaces,silicon carbide, ZnO, GaAs, GaP, GaN, Ge, InP, iron, steel, stainlesssteel, molybdenum, alumina, magnesia and titania or combinationsthereof.
 4. The carbon nanotube structure of claim 1 wherein thecatalytic surface comprises catalytic islands thereon comprising one ormore metal selected from the group consisting of Group VIII metals,Group VIb metals, Group Vb metals and Re.
 5. The carbon nanotubestructure of claim 1 wherein catalytic islands are formed on thecatalytic surface, and wherein the catalytic islands are separated by anaverage distance of 30 nm to 100 nm.